In wireless communication system utilizing unlicensed spectra, like WLAN (Wireless Local Area Network), before transmitting data, a node usually has to perform a Listen-Before-Talk (LBT) procedure to determine whether the carrier (of the unlicensed spectrum used) it wants to access is available for use. In contrast, many mobile communication systems used licensed spectra, and do not utilize such LBT procedures, as the access to the carriers is strongly controlled by the network, respectively base station. However, current developments in mobile telecommunications allow the use of unlicensed spectra for increased data throughput, in particular in addition to licensed spectra, e.g. in the context of carrier aggregation (CA).
In typical deployments of WLAN, e.g. according to Wifi, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel or carrier is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel or carrier is declared as idle. In case the channel or carrier is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several APs (Access Points, e.g. WLAN node) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same carrier or frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel or carrier in time, and the throughput for the individual APs may be severely degraded. A general illustration of this listen before talk (LBT) mechanism is shown in FIG. 1.
After a Wifi station (as an example of a WLAN node) A transmits a data frame to a station B, station B shall transmit an ACK frame (Acknowledgement frame) back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit, first performs a CCA (Clear Channel Assessment) by sensing the medium (carrier or channel) for a fixed duration DIFS. If the medium is idle, then the station assumes that it may take ownership of the medium (comprising accessing and/or transmitting) and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
To further prevent a station from occupying the channel or carrier continuously and thereby prevent other stations from accessing the channel or carrier, it is required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
The PIFS is used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it can be used by STAs operating under PCF (Point coordination Function), to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the PC shall sense the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the PC shall transmit a Beacon frame containing the CF Parameter Set element and a delivery traffic indication message element.
In the above basic protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff window, CWmin, is set in the IEEE specs. Note that collisions can still happen even with this random backoff protocol when they are many stations contending for the channel access. Hence, to reduce continuous collisions, the backoff window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specs. When a station succeeds in a transmission without collision, it resets its random backoff window size back the default value CWmin.
In the context of the expected increase in use of wireless devices and, in particular, the use of unlicensed spectra for mobile communication, the LBT procedure used for WLAN is unlikely to ensure fair access for all nodes wanting access to certain carriers or channels, in particular in an unlicensed spectrum.